SHADOW CHILDREN SERIES by MARGARET PETERSON HADDIX

These series of books is about a futuristic, overpopulated, resource-deprived Earth, and the effects of the government's attempts to quell overpopulation. The Population Police enforce the government's Population Law, killing or imprisoning "shadow children," any third child in a family. (synopsis borrowed from Wikipedia)

Book One - Among the Hidden

Book Two - Among the Imposters

Book Three - Among the Betrayed

Book Four - Among the Barons

Book Five - Among the Brave

Book Six - Among the Enemy

Book Seven - Among the Free

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